1. Field of the Invention
The present invention relates to a semiconductor laser device, a semiconductor laser module, and a semiconductor laser device fabrication method.
2. Discussion of the Background
In recent years, as the Internet becomes more popular, connections between LAN networks have rapidly increased, and therefore a sharp increase of data traffic has been observed, posing a problem in terms of communication performance. Thus, a DWDM (Dense-Wavelength Division Multiplexing) transmission system to prevent the communication performance from deteriorating has been proposed.
The DWDM transmission system realizes large-capacity data transmission, two orders of magnitude greater per fiber than a conventional system, by superimposing a plurality of optical signals with various wavelengths. A signal light source or amplification light source used for the DWDM transmission system is required to control an oscillation wavelength with a high accuracy at high optical output while preventing a thermal saturation of the semiconductor laser element. Conventionally, a semiconductor laser device achieves these results by monitoring and controlling a temperature of the laser element. More specifically, for preventing the wavelength of the electromagnetic wave from becoming unstable and the semiconductor laser element from becoming thermally saturated, a thermistor is set to measure the temperature of the semiconductor laser element that outputs a laser beam, and control the temperature of the semiconductor laser element by a temperature control element, such as a Peltier element.
FIG. 16 is a perspective view that shows a schematic structure of a conventional semiconductor laser device. In FIG. 16, the semiconductor laser device has a submount 102 made of AlN having an insulating property and a high heat conductivity. The submount 102 is set on a carrier 101 made of CuW. A semiconductor laser element 103 that outputs a laser beam L100 having a predetermined wavelength is set on the submount 102. Moreover, a submount 104 made also of AlN is set on the carrier 101, and a thermistor 105 that measures the temperature of the semiconductor laser element 103 is set onto the submount 104.
Therefore, the submount 102 secures the insulation of the semiconductor laser element 103 and functions as a heat sink of the semiconductor laser element 103. The submount 102 is connected to a CuW base (not shown) connected by AuSn solder below the carrier 101 and to a Peltier module (not shown) disposed below the base for controlling the temperature of the semiconductor laser element 103 in accordance with the temperature detected by the thermistor 105. The thermistor 105 is also insulated from the carrier 101 by the submount 104, similarly as the semiconductor laser element 103. The thermistor 105 indirectly detects the temperature of the semiconductor laser element 103 through the submount 102, carrier 101, and submount 104, respectively.
For the above DWDM transmission system, it is desired to increase the output of the laser beam of the signal light source in order to increase a distance between repeaters necessary for connecting various LANs. Moreover, to improve the amplification of an optical fiber amplifier, it is desired to increase the output of the semiconductor laser device used in an excitation light source. To increase the output of the laser beam of the semiconductor laser device, it is known that increasing the cavity length of the semiconductor laser element 103 and improving the radiation characteristic of the semiconductor laser element achieve this result. However, because the increase of the cavity length causes the increase of heat generation, an improvement of the radiation characteristic is the preferred method for increasing the output of the laser element. One way to improve the radiation efficiency and output power of the laser element 103 is by forming the submount 102 from a material such as diamond, which has a high radiation characteristic, and arranging the semiconductor laser element 103 in a junction-down configuration.
However, the diamond made submount 102 causes reliability problems for the laser element 103. Specifically, a repeated heat generation by the semiconductor laser element 103 and cooling by the submount 102 make the characteristics of the semiconductor laser element 103 deteriorate, and the element 103 may break due to the strain produced by the difference between linear expansion coefficients of the diamond submount 102 and the semiconductor laser element 103. In other words, using a diamond submount 102 for improving the thermal characteristics of the laser device causes an undesirable mechanical strain between the submount 102 and the laser element 103.